This study has been designed to investigate the subgroup differences in the meningococal C polysaccharide and the biologic significance of these differences in human and animal systems. Identification of these subgroup antigens will be accomplished using hemagglutination, hemagglutination inhibition, immunodiffusion, and bactericidal systems. Epidemiologic studies of the prevalence and clinical significance of these subgroup determinants will be performed on large number of group C meninggococci with an immunofluorescent analysis specific for the capsular subgroup determinants. The human immune response to group C meningococcal vaccines produced from organisms containing the subgroup antigens will be studied in hemagglutination and bactericidal systems. Investigations will also be undertaken with group C organisms which are resistant to the bactericidal action of antibody to the capsular C polysaccharide.